Abstract
The reaction of diphenyl nitrilimine (NI) with methyl 1-methyl-allenoate yielding a spirobipyrazoline has been studied within molecular electron density theory (MEDT) at the MPWB1K/6-311G(d) computational level in dichloromethane. This reaction is a domino process that comprises two consecutive 32CA reactions with the formation of a pyrazoline intermediate. Analysis of the relative Gibbs free energies indicates that both 32CA reactions are highly regioselective, the first one being also completely chemoselective, in agreement with the experimental outcomes. The geometries of the TSs indicate that they are associated to asynchronous bond formation processes in which the shorter distance involves the C1 carbon of diphenyl NI. Despite the zwitterionic structure of diphenyl NI, the appearance of a pseudoradical structure at the beginning of the reaction path, with a very low energy cost, suggests that the 32CA reaction between diphenyl NI, a strong nucleophile, and the allenoate, a moderate electrophile, should be mechanistically considered on the borderline between pmr-type and cb-type 32CA reactions, somewhat closer to the latter.
Highlights
The present MEDT study has been divided in four sections: (i) in Section 3.1, the electronic structure of diphenyl NI 1 and allenoate 4 is analysed by means of the topological analysis of the Electron Localisation Function [62] (ELF); (ii) in Section 3.2, an analysis of the Conceptual DFT (CDFT) reactivity indices of the reagents is carried out; (iii) in Section 3.3, the reaction paths associated with the domino reaction between diphenyl NI 1 and allenoate 4 are investigated; and (iv) in Section 3.4, the formation of the C–C and C–N single bonds along the most favourable reaction path associated with the 32CA reaction of diphenyl NI 1 with allenoate 4 is characterized
An appealing procedure that provides a straightforward connection between the electron density distribution and the chemical structure is the quantum chemical analysis of Becke and Edgecombe’s ELF [62]
In order to characterize the electronic structure of diphenyl NI 1 and, to establish its reactivity in 32CA reactions, a topological analysis of the ELF of this TAC, as well as of allenoate 4, was first performed
Summary
Topological analysis of the ELF of dimethyl NI 13 shows the presence of a V(C1) monosynaptic basin integrating 1.51e, characterising the C3 carbon as a carbenoid center, while the C1–N2 and C1–C(Me) bonding regions have been depopulated by 0.63 and 0.96 e, respectively, with respect to the equivalent regions of diphenyl NI 1. Note that the analysis of the nucleophilic P−k Parr functions at the reactive sites of diphenyl NI 1 predicts this reaction path to be less favourable than its regioisomeric one; (iii) this 32CA reaction is completely chemoselective as TS21 is 6.4 kcal·mol−1 above TS11; (iv) formation of pyrazoline 6 is strongly exothermic by 69.8 kcal·mol−1; and (v) these energy results rule out the formation of pyrazoline 14 along this domino reaction, in complete agreement with the experimental observation of pyrazoline 6 as the only transient species in the reaction media [40].
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